1. Field of the Invention
The invention is related to the field of nuclear magnetic resonance ("NMR") apparatus and measuring techniques. More specifically, the invention is related to well logging apparatus and measuring techniques for NMR measurement within earth formations penetrated by a wellbore.
2. Description of the Related Art
NMR well logging instruments typically include a permanent magnet to induce a static magnetic field in the earth formations and a transmitting antenna, positioned near the magnet and shaped so that a pulse of radio frequency (RF) power conducted through the antenna induces an RF magnetic field in the earth formation. The RF magnetic field is generally orthogonal to the static magnetic field. After an RF pulse, voltages are induced in a receiving antenna positioned near the magnet by precessional rotation of nuclear spin axes of hydrogen or other nuclei about the static magnetic field. The receiving antenna is typically connected to a receiver which detects and measures the induced voltages.
In a typical NMR measurement set a sequence of RF pulses is applied and a sequence of voltages is measured. The magnitudes of the voltages and the rates at which the voltages vary are related to certain petrophysical properties of the earth formation. These properties can include the fractional volume of pore space, the fractional volume of mobile fluid filling the pore spaces of the earth formations and other petrophysical parameters. Methods and measurement techniques for using NMR measurements to determine the fractional volume of pore space, the fractional volume of mobile fluid and other petrophysical parameters are described, for example, in Spin Echo Magnetic Resonance Logging: Porosity and Free Fluid Index Determination, M. N. Miller et al, Society of Petroleum Engineers paper no. 20561, Richardson, Tex., 1990 and in Field Test of an Experimental Pulsed Nuclear Magnetism Tool, C. E. Morriss et al, SPWLA Transactions 1993, paper GGG.
One NMR well logging instrument is described, for example, in U. S. Pat. No. 3,597,681 issued to Huckbay et al. A drawback to the apparatus disclosed in the Huckbay et al '681 patent is that a region of unidirectional magnetic field (the "sensitive region") induced in the formation by the magnet is not homogeneous along the wellbore axis. Logging tools typically must be able to move axially through the wellbore while performing measurements. During the time needed to make a typical NMR measurement set the sensitive region will change its position before the measurement set can be completed, leading to error in the measurements. Another drawback to the apparatus disclosed in the Huckbay et al '681 patent is that a significant part of the NMR signal can originate within a fluid ("drilling mud") filling the wellbore. Another drawback to the apparatus disclosed in the Huckbay et al '681 patent is that the RF magnetic field decreases in amplitude with respect to the third power of the distance between the antenna and the sensitive region, as the antenna can be modeled as the equivalent of a three dimensional magnetic dipole. Such an antenna is proximally coupled to only a small part of the unidirectional static magnetic field. This results in an extremely low signal-to-noise ratio. Another drawback to the apparatus disclosed in the Huckbay et al '681 patent is that the antenna is subjected to a very high static magnetic field strength and will have an unacceptably large amount of magnetoacoustic ringing as a result.
Another type of NMR well logging apparatus is described in U.S. Pat. No. 4,350,955 issued to Jackson et al. The apparatus disclosed in the Jackson et al '955 patent includes permanent magnets configured to induce a static magnetic field in the earth formations which has a toroidal volume of substantially uniform magnetic field strength. A particular drawback to the apparatus disclosed in the Jackson et al '955 patent is that the thickness of the toroidal volume is very small relative to typical rates of axial motion of well logging tools. Well logging tools, in order to be commercially useful, typically must be able to be moved axially through the wellbore at rates not less than about ten feet per minute. The length of time needed to make a typical NMR measurement set can be as long as several seconds. The NMR logging tool is therefore likely to move a substantial distance during a measurement cycle. Measurements made by the apparatus disclosed in the Jackson et al '955 patent are therefore subject to error.
Another drawback to the apparatus disclosed in the Jackson et al '955 patent is that it does not eliminate NMR signal originating within the fluid filling the wellbore. A still further drawback to the apparatus disclosed in the Jackson et al '955 patent is that the toroidally shaped static magnetic field is subject to changes in field strength as the instrument is subjected to changes in ambient temperature and variances in the earth's magnetic field. The antenna in the Jackson et al '955 apparatus is tuned to a single frequency. If the field strength of the static magnetic field in the toroidal volume changes, the antenna may no longer be sensitive to NMR signals originating within the toroidal volume. Using the apparatus in Jackson et al '955, it is impractical to compensate the frequency of the RF magnetic field for changes in the static magnetic field strength within the toroidal volume.
Additional drawbacks to the apparatus disclosed in the Jackson et al '955 patent are as follows. Since the magnet pole pieces are in opposed polarity to each other, there is a significant demagnetizing effect which requires a permanent magnet material having high coercive force. This requirement is opposite to the strong residual magnetization and high temperature stability of magnetic properties required of the permanent magnet. The magnet pole pieces are significantly spaced apart and far from the magnetic field's homogeneous region, which makes the usage of the permanent magnet material less cost-effective. Low antenna efficiency is a result of low electro-magnetic coupling between the antenna and the earth formation at resonance. The antenna is located in a relatively strong magnetic field, which stimulates strong magnetoacoustic ringing in the antenna. Because it uses a homogeneous magnetic field, any changes in the orientation of the apparatus with respect to the earth magnetic's field can result in a significant disturbance to the homogeneity of the static magnetic field. Furthermore, some techniques for diffusion measurement require a substantial magnetic field gradient in the static magnetic field, which are made impossible by the homogeneous static magnetic field of the Jackson et al '955 apparatus.
Another type of NMR well logging apparatus is described in U.S. Pat. No. 4,717,876 issued to Masi et al. The apparatus disclosed in the Masi et al '876 patent has a "toroidal" static magnetic field providing improved homogeneity in the toroidal region as compared to apparatus in the Jackson et al '955 patent, but the Masi et al apparatus has basically the same drawbacks as the Jackson et al apparatus.
Another type of NMR well logging apparatus is described in U.S. Pat. No. 4,629,986 issued to Clow et al. This apparatus provides improved signal-to-noise ratio, when compared to the apparatus of the Jackson et al '955 patent, by including a high magnetic permeability ferrite in the antenna. Increased stability is achieved by having a static magnetic field gradient as part of the static magnetic field in the sensitive region. However, the apparatus disclosed in the Clow et al '986 patent has the following drawbacks. Since the magnetic properties of the permanent magnet material are temperature dependent, the sensitive region is not stable in shape and field intensity. The sensitive region is only a couple of inches long in the longitudinal direction, which requires this tool to be practically motionless during an NMR measurement cycle. Magnet pole pieces are significantly spaced apart and far from the homogeneous field region, which makes the usage of permanent magnet material not cost-effective. The antenna is located in a relatively strong static magnetic field, which stimulates magnetoacoustic ringing in the antenna. The high magnetic permeability ferrite in the antenna is located in a relatively strong magnetic field which may saturate the ferrite and reduce its efficiency. Soft ferrite in a static magnetic field is also a strong source of magnetostrictive ringing following each RF pulse. In the magnet arrangement disclosed by Clow et al the demagnetizing field is substantially strong, which requires magnet material having a high coercive force. This requirement is opposite to the strong residual magnetization and high temperature stability of magnetic properties required of the permanent magnet. The static magnetic field in the earth formation at resonance is only about 10 Gauss and rotates 360.degree. in a plane perpendicular to the wellbore axis. For this level of static magnetic field, the earth's magnetic field of about 0.5 Gauss presents a significant disturbance to the static field induced by the magnet.
Another type of NMR well logging apparatus is described in U.S. Pat. No. 4,717,878 issued to Taicher et al provides azimuthal resolution with respect to the wellbore axis and reduction of spurious signal from wellbore fluid. However, the apparatus disclosed in the Taicher et al '878 patent has the following drawbacks. Since the magnetic properties of the permanent magnet material are temperature dependent, the sensitive region is not stable in shape and field intensity. The antenna is located in a relatively strong magnetic field, which stimulates magnetoacoustic ringing in the antenna. In the magnet arrangement disclosed by Taicher et al, the demagnetizing field is very strong, which requires a magnet material having high coercive force. This requirement is opposite to the strong residual magnetization and high temperature stability of magnetic properties required of the permanent magnet. Due to the disadvantages of the foregoing NMR well logging instruments, none of them generally has become commercially accepted.
One NMR well logging apparatus which has become commercially accepted is described in U.S. Pat. No. 4,710,713 issued to Taicher et al. The apparatus disclosed in the Taicher et al '713 patent includes a substantially cylindrical permanent magnet assembly which induces a static magnetic field having substantially uniform field strength within an annular cylindrical volume. The apparatus disclosed in the Taicher et al '713 patent, however, has several drawbacks. First, the antenna induces an RF magnetic field in the formations surrounding the tool which decreases in strength as the square of the radial distance from the axis of the magnet. Moreover, a significant portion of the RF energy can be lost in an electrically conductive fluid in the wellbore. Because the signal-to-noise ratio of NMR measurements made in a gradient magnetic field is typically related to the strength of the RF magnetic field, the apparatus disclosed in the Taicher et al '713 has large electrical power requirements and can have difficulty obtaining measurements having sufficient signal-to-noise ratio at substantial radial distances from the axis of the instrument.
Another drawback to the instrument described in the Taicher et al '713 patent is that the optimum design of the magnet and the RF antenna requires the resonance conditions to be met at a relatively high frequency to obtain a suitable signal-to-noise ratio. Since the RF energy losses in the fluid in the wellbore (if it is conductive) are proportional to the square of the frequency, the operation of the Taicher et al '713 patent is generally restricted to operating in a low electrical conductivity wellbore fluid.
Another drawback to the apparatus described in the Taicher et al '713 patent is that the optimum design of the magnet and the RF antenna requires the sensitive volume to be at about 12 inches to 15 inches in diameter in order to provide acceptable signal-to-noise ratio. Many wellbores are inclined from vertical and logging tools cannot be ideally centralized in such wellbores. Moreover, the wellbore can sometimes have a very large internal diameter as a result of "washouts" or similar effects known in the art. For wellbores having a nominal diameter of larger than about 10 inches, and particularly those highly inclined form vertical, the sensitive volume of this apparatus may be positioned at least partially within the wellbore itself rather than wholly within the earth formation leading to errors in the measurement.
Yet another drawback to the apparatus described in the Taicher et al '713 patent is that the antenna is located in a relatively strong magnetic field, which is perpendicular to a direction of RF current flow in the transmitting antenna and, therefore, stimulates magnetoacoustic ringing in the transmitting antenna.
Another commercially accepted NMR logging apparatus is described in U.S. Pat. No. 5,055,787 issued to Kleinberg et al. This logging instrument includes permanent magnets arranged to induce a magnetic field in the earth formation having substantially zero static magnetic field gradient within a predetermined sensitive volume. The magnets are arranged in a portion of the tool housing which is typically placed in contact with the wall of the wellbore. The antenna in this logging instrument is positioned in a recess located external to the tool housing, enabling the tool housing to be made from a high strength material such as steel. A drawback to the instrument described in the Kleinberg et al '787 patent is that its sensitive volume is only about 0.8 cm away from the tool surface and extends only to about 2.5 cm radially outward from the tool surface. Measurements made by this instrument are therefore subject to large error caused by, among other things, roughness in the wall of the wellbore, by deposits of the solid phase of the drilling mud (called "mudcake") onto the wall of the wellbore in any substantial thickness, and by the fluid content of the formation in the "invaded zone" (typically defined as the zone in which the liquid phase of the drilling mud infiltrates the pore spaces of the formation proximal to the wellbore).
Another drawback to the apparatus disclosed in the Kleinberg et al '787 patent relates to the magnet material used. Since the magnet pole pieces are opposed each other, there is a strong demagnetizing effect which requires a magnet material having a high coercive force. This requirement is opposite to the strong residual magnetization and high temperature stability of magnetic properties required of the permanent magnet.
All of the prior art NMR well logging instruments described herein typically have antennas for generating the RF magnetic field and for receiving the NMR signals which are substantially the same length as the axial extent of the static magnetic field. A drawback to prior art NMR apparatus having such antenna dimensions is that measurements made in which the instrument is moving are subject to significant error. The first source of error is that the RF magnetic field may be generated in a region different from that which is completely "prepolarized" by the static magnetic field. A second source of error is that the receiving antenna may be sensitive to an axial region which is different from the axial region in which the NMR signal is likely to originate, as the instrument is axially moved during measurement.
Prior art NMR well logging instruments have a common drawback as explained, for example, in U.S. Pat. No. 5,332,967 issued to Shporer. This drawback is related to a significant phase shift of the NMR signal, which leads to significant distortion of the signal height and may even lead to a complete disappearance of the signal, when the logging apparatus is moving in a direction along a static magnetic field magnitude gradient. This signal reduction may become even more pronounced when the speed of motion of the instrument is variable and uncontrolled. Causes of variation in the speed of motion of a logging instrument are well known in the art.